Highly Stable Self-Regenerating Organic Multi-Redox Systems derived from Bicyclic (Alkyl)(amino)carbenes (BICAACs)

An extended class of organic multi-redox systems was derived from bicyclic(alkyl)amino carbenes (BICAACs). The highly-conjugated system undergoes a total of 4 redox events spanning a 1.8 V redox range. These organic compounds exhibited four different stable redox states (dication, radical cation, neutral and radical anion), and all of them were characterized either by single crystal X-ray study and/or various spectroscopic studies. Three of the four redox states are stable to air and moisture. The availability of stable multiple redox states demonstrated promise towards their efficacy in the symmetric H-cell charge/discharge cycling. Among various redox states, the dication/neutral state works efficiently and continuously for 1500 cycles in 2e- charge/discharge process outside glovebox in commercially available DMF with minimum capacity loss (retaining nearly 90 % Coulombic efficiency). Surprisingly, the efficiency of the redox cycle was retained even if the system was exposed to air for 30 days when it slowly regenerated to the initial deep blue radical cation, and it exhibited another 100 charge/discharge cycles with a minimal capacity loss. Such a stable H-cell cycling ability is not well known among organic molecule-based systems.

Hexaguanidino-Triptycenes and Triphenylenes: Electronic Coupling in Molecules Containing Three Redox-Active o-Diguanidinobenzene Units Connected either Directly or Interacting Through Homoconjugation

Novel redox-active hexaguanidine molecules with multiple redox states were synthesized by connecting three o-diguanidinobenzene units. In 2,3,6,7,14,15-hexaguanidino-triptycenes, the three redox-active o-diguanidinobenzene units are connected through C-C bonds to the sp3 -hybridized bridgehead C atoms, and in 2,3,6,7,10,11-hexaguanidino-triphenylenes they are directly connected. The connectivity difference leads to different electronic coupling between the three redox-active o-diguanidinobenzene units, with homoconjugation being present in the triptycene, but not in the triphenylene compounds. Motivated by the appearance of an intense low-energy electronic transition, we especially analysed the effect of homoconjugation on the electronic structure and charge delocalization in the dicationic redox state of the triptycene derivatives. Then, several trinuclear high-spin cobalt (and copper) complexes were synthesized with the triphenylene and triptycene ligands, and the magnetic coupling and redox properties analysed. By choice of the coligands (hexafluoroacetylacetonate, trifluoroacetylacetonate and acetylacetonate), oxidation could be switched between metal- and ligand-centered redox events, leading to drastic changes in the magnetic or optical properties, especially as a consequence of homoconjugation in the triptycene derivatives.

Intramolecular Through-Space Double-Electron Transfer Between A Pair of Redox-Active Guanidine Units Aligned by Dithiolate Bridges

Using unconventional synthesis protocols, two redox-active triguanidine units are connected by a dithiolate bridge, aligning the two redox-active units in close proximity. The reduced, neutral and the tetracationic redox states with two dicationic triguanidine units are isolated and fully characterized. Then, the dicationic redox states are prepared by mixing the neutral and tetracationic molecules. At low temperatures, the dications are diamagnetic (singlet ground state) with two different triguanidine units (neutral and dicationic). At room temperature, the triplet state with two radical monocationic triguanidine units is populated. At low temperature (210 K), chemical exchange by intramolecular through-space electron-transfer between the two triguanidine units is evidenced by EXSY NMR spectroscopy. Intramolecular through-space transfer of two electrons from the neutral to the dicationic triguanidine unit is accompanied by migration of the counterions in opposite direction. The rate of double-electron transfer critically depends on the bridge. No electron-transfer is measured in the absence of a bridge (in a mixture of one dicationic and one neutral triguanidine), and relatively slow electron transfer if the bridge does not allow the two triguanidine units to approach each other close enough. The results give detailed, quantitative insight into the factors that influence intramolecular through-space double-electron-transfer processes.

Organic Four-Electron Redox Systems Based on Bipyridine and Phenanthroline Carbene Architectures

Novel organic redox systems that display multistage redox behaviour are highly sought-after for a series of applications such as organic batteries or electrochromic materials. Here we describe a simple strategy to transfer well-known two-electron redox active bipyridine and phenanthroline architectures into novel strongly reducing four-electron redox systems featuring fully reversible redox events with up to five stable oxidation states. We give spectroscopic and structural insight into the changes involved in the redox-events and present characterization data on all isolated oxidation states. The redox-systems feature strong UV/Vis/NIR polyelectrochromic properties such as distinct strong NIR absorptions in the mixed valence states. Two-electron charge-discharge cycling studies indicate high electrochemical stability at strongly negative potentials, rendering the new redox architectures promising lead structures for multi-electron anolyte materials.

A Bottleable Imidazole-Based Radical as a Single Electron Transfer Reagent

Reduction of 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenyl-1H-imidazol-3-ium chloride (1) resulted in the formation of the first structurally characterized imidazole-based radical 2. 2 was established as a single electron transfer reagent by treating it with an acceptor molecule tetracyanoethylene. Moreover, radical 2 was utilized as an organic electron donor in a number of organic transformations such as in activation of an aryl-halide bond, alkene hydrosilylation, and in catalytic reduction of CO₂ to methoxyborane, all under ambient temperature and pressure.

Iminophosphorano-Substituted Bispyridinylidenes: Redox Potentials and Substituent Constants from Tolman Electronic Parameters

Bispyridinylidenes (BPYs) have emerged as an important class of neutral organic electron donors, with redox potentials that vary widely with choice of substituent. Methods to predict the effect of substitution on the redox potential are therefore highly desirable. Here we show that the redox potential of BPYs featuring iminophosphorano substituents (R₃ P=N-), which represent the most reducing class of BPYs, can be predicted based on the well-known Tolman electronic parameter (TEP) for the respective phosphine fragment (R₃ P). Moreover, building on earlier work relating redox potentials to Hammett-type substituent constants, it is now possible to quantitatively predict σ_p ⁺ values for iminophosphorano substituents from TEP values. These results provide a path for precisely tailoring redox potentials of iminophosphorano-substituted BPYs, but also give quantitative descriptors for how these highly versatile iminophosphorano substituents can impact the properties of any molecular scaffold.

Orthoamide und Iminiumsalze, IIC. Darstellung von N-(ω-Ammonioalkyl)-N,N′,N′,N″,N″-peralkylierten Guanidiniumsalzen und N-(ω-Aminoalkyl)-N′,N′,N″,N″-tetramethylguanidinen

N,N,N′,N′-Tetraalkylchlorformamidiniumchlorides 1a, b react with ω-dimethylaminoalkylamines 19, 20 to give mixtures of N-(ω-dimethylammonioalkyl)-guanidinium salts 12, 13 and N-(ω-dimethylaminoalkyl)-guanidinium salts 21, 22. These mixtures are transformed to mixtures of the ureas 15, 17 and N-(ω-dimethylaminoalkyl)-guanidines 23, 25 on treatment with aqueous sodium hydroxide. The reaction of N-(3-dimethylammoniopropyl)-guanidin 25a with dimethylsulfate in a molar ratio of 1:1 delivers a mixture of the N-(3-dimethylaminopropyl)-N,N,N′,N′,N″,N″-pentamethyl-guanidinium salt 29a and the N-(3-dimethylammoniopropyl)-N,N′,N′,N″,N″-pentamethyl-guanidinium-bis (methylsulfate) 33a. The action of dimethylsulfate on the guanidines 23a, 25a in a molar ratio of 2:1 affords the bisquarternary salts 32a, 33a. Alkylating reagents as methyliodide, benzylbromide, allylbromide and chloroacetonitrile attack N-(2-dimethylaminoethyl)-N′,N′,N″,N″-tetraethylguanidine (23b) in a molar ratio of 1:1 cleanly at the dimethylaminoethylgroup to give the ammonium salts 30a–d. As a strong base the guanidine 23b dehydrochlorinates β-Chlorpropionitrile and chloroacetone under formation of the guanidinium salt 21c. In contrast to this the reaction of ethyl bromoacetate with the N-(2-dimethylaminoethyl)guanidine 23b occurs at the guanidinogroup giving the guanidinium salt 28c. The methylation of the guanidinium chlorides 21a, 22a with dimethyl sulfate affords the bis-quaternary salts 35b, 36b with mixed anions. From the heterocyclic guanidines 14, 16 and the alkylating reagents benzylbromide and ethyl bromoacetate the heterocyclic guanidinium salts 37a, b, 39a, b can be obtained. The reactions with ethyl chloroformiate proceed in an analogous way giving the guanidinium salts 37c, 39c. The N-alkyl-N,N,N′,N′-tetramethyl-(3-ureidopropyl)guanidinium salts 41a, b can be prepared from the N′,N′,N″,N″-tetramethyl-N′′-(3-ureidopropyl) guanidine 17a and the alkylating compounds dimethyl sulfate and benzyl bromide. Several compounds obtained that way were transformed to the corresponding tetraphenyloborates and bis(tetraphenylborates), respectively.

1,2,4,5-Tetrakis(tetramethylguanidino)-3,6-diethynyl-benzenes: Fluorescent Probes, Redox-Active Ligands and Strong Organic Electron Donors

In this work, the change of reactivity induced by the introduction of two para-ethynyl substituents (CCSi(iPr)3 or CCH) to the organic electron-donor 1,2,4,5-tetrakis(tetramethylguanidino)-benzene is evaluated. The redox-properties and redox-state dependent fluorescence are evaluated, and dinuclear CuI and CuII complexes synthesized. The Lewis-acidic B(C6 F5 )3 substitutes the proton of the ethynyl -CCH groups to give new anionic -CCB(C6 F5 )3 - substituents, leading eventually to a novel dianionic strong electron donor in its diprotonated form. Its two-electron oxidation with dioxygen in the presence of a copper catalyst yields the first redox-active guanidine that is neutral (instead of cationic) in its oxidized form.

1,2,5,6-Tetrakis(guanidino)-Naphthalenes: Electron Donors, Fluorescent Probes and Redox-Active Ligands

New redox-active 1,2,5,6-tetrakis(guanidino)-naphthalene compounds, isolable and storable in the neutral and deep-green dicationic redox states and oxidisable further in two one-electron steps to the tetracations, are reported. Protonation switches on blue fluorescence, with the fluorescence intensity (quantum yield) increasing with the degree of protonation. Reactions with N-halogenosuccinimides or N-halogenophthalimides led to a series of new redox-active halogeno- and succinimido-/phthalimido-substituted derivatives. These highly selective reactions are proposed to proceed via the tri- or tetracationic state as the intermediate. The derivatives are oxidised reversibly at slightly higher potentials than that of the unsubstituted compounds to dications and further to tri- and tetracations. The integration of redox-active ligands in the transition-metal complexes shifts the redox potentials to higher values and also allows reversible oxidation in two potentially separated one-electron steps.

Aromaticity and sterics control whether a cationic olefin radical is resistant to disproportionation

We elucidate why some electron rich-olefins such as tetrathiafulvalene (TTF) or paraquat (1,1'-dimethyl-4,4'-bipyridinylidene) form persistent radical cations, whereas others such as the dimer of N,N'-dimethyl benzimidazolin-2-ylidene (benzNHC) do not. Specifically, three heterodimers derived from cyclic (alkyl) (amino) carbenes (CAAC) with N,N'-dimethyl imidazolin-2-ylidene (NHC), N,N'-dimethyl imidazolidin-2-ylidene (saNHC) and N-methyl benzothiazolin-2-ylidene (btNHC) are reported. Whereas the olefin radical cations with the NHC and btNHC are isolable, the NHC compound with a saturated backbone (saNHC) disproportionates instead to the biscation and olefin. Furthermore, the electrochemical properties of the electron-rich olefins derived from the dimerization of the saNHC and btNHC were assessed. Based on the experiments, we propose a general computational method to model the electrochemical potentials and disproportionation equilibrium. This method, which achieves an accuracy of 0.07 V (0.06 V with calibration) in reference to the experimental values, allows for the first time to rationalize and predict the (in)stability of olefin radical cations towards disproportionation. The combined results reveal that the stability of heterodimeric olefin radical cations towards disproportionation is mostly due to aromaticity. In contrast, homodimeric radical cations are in principle isolable, if lacking steric bulk in the 2,2' positions of the heterocyclic monomers. Rigid tethers increase accordingly the stability of homodimeric radical cations, whereas the electronic effects of substituents seem much less important for the disproportionation equilibrium.

Divergent 2-Chloroquinazolin-4(3H)-one Rearrangement: Twisted-Cyclic Guanidine Formation or Ring-Fused N-Acylguanidines via a Domino Process

A highly efficient 2-chloroquinazolin-4(3H)-one rearrangement was developed that predictably generates either twisted-cyclic or ring-fused guanidines in a single operation, depending on the presence of a primary versus secondary amine in the accompanying diamine reagent. Exclusive formation of twisted-cyclic guanidines results from pairing 2-chloroquinazolinones with secondary diamines. Use of primary amine-containing diamines permits a domino quinazolinone rearrangement/intramolecular cyclization, gated through (E)-twisted-cyclic guanidines, to afford ring-fused N-acylguanidines. This scalable, structurally tolerant transformation generated 55 guanidines and delivered twisted-cyclic guanidines with robust plasma stability and an abbreviated total synthesis of an antitumor ring-fused guanidine (4 steps, 55 % yield).

Hierarchically Structured Two-Dimensional Bimetallic CoNi-Hexaaminobenzene Coordination Polymers Derived from Co(OH)2 for Enhanced Oxygen Evolution Catalysis

Conjugated coordination polymers (CPs) with designable and predictable structures have drawn tremendous attention in recent years. However, the poor electrical conductivity and low structural stability seriously restrict their practical applications in electronic devices. Herein, the rational design and synthesis of a hierarchically structured 2D bimetallic CoNi-hexaaminobenzene CPs derived from Co(OH)₂ are reported as an efficient oxygen evolution reaction (OER) self-supported electrode. The as-obtained electrode possesses high electrochemical surface area and intrinsic activity, exhibiting high electrochemical catalytic activity, favorable reaction kinetics performance, and strong durability compared with those of the powder catalysts. As a result, the electrode delivers low overpotential of 219 mV @ 10 mA cm^-2 and Tafel slope of 42 mV dec^-1 as well as 91.3% retention of current density after 24 h of reaction time. The results of density functional theory computations reveal that the synergistic effect of Co and Ni plays an important role in OER. This work not only presents a strategy to fabricate advanced self-supported electrodes with abundant and dense active sites, but also promotes the development of conjugated CPs for electrocatalysis.

Neutral Organic Super Electron Donors Made Catalytic

Neutral organic super electron donors (SEDs) display impressive reducing power but, until now, it has not been possible to use them catalytically in radical chain reactions. This is because, following electron transfer, these donors form persistent radical cations that trap substrate-derived radicals. This paper unlocks a conceptually new approach to super electron donors that overcomes this issue, leading to the first catalytic neutral organic super electron donor.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

Efficient n-Doping and Hole Blocking in Single-Walled Carbon Nanotube Transistors with 1,2,4,5-Tetrakis(tetramethylguanidino)ben-zene

Efficient, stable, and solution-based n-doping of semiconducting single-walled carbon nanotubes (SWCNTs) is highly desired for complementary circuits but remains a significant challenge. Here, we present 1,2,4,5-tetrakis(tetramethylguanidino)benzene (ttmgb) as a strong two-electron donor that enables the fabrication of purely n-type SWCNT field-effect transistors (FETs). We apply ttmgb to networks of monochiral, semiconducting (6,5) SWCNTs that show intrinsic ambipolar behavior in bottom-contact/top-gate FETs and obtain unipolar n-type transport with 3-5-fold enhancement of electron mobilities (approximately 10 cm² V^-1 s^-1), while completely suppressing hole currents, even at high drain voltages. These n-type FETs show excellent on/off current ratios of up to 10⁸, steep subthreshold swings (80-100 mV/dec), and almost no hysteresis. Their excellent device characteristics stem from the reduction of the work function of the gold electrodes via contact doping, blocking of hole injection by ttmgb²⁺ on the electrode surface, and removal of residual water from the SWCNT network by ttmgb protonation. The ttmgb-treated SWCNT FETs also display excellent environmental stability under bias stress in ambient conditions. Complementary inverters based on n- and p-doped SWCNT FETs exhibit rail-to-rail operation with high gain and low power dissipation. The simple and stable ttmgb molecule thus serves as an example for the larger class of guanidino-functionalized aromatic compounds as promising electron donors for high-performance thin film electronics.